Season 2 Episode 11 Or The One With The Terrible Hats, if you prefer. For quite a while Carol and Susan's relationship was a footnote in Ross' tragic little life - the punchline for his inability to form real relationships with women and a sign that, sadly, he was something of a loser. The idea that he brought the pair together by suggesting a threesome was just a genius cherry on top. But then in the middle of season 2, the pair got married - a ceremony that Ross played a central part in - giving fans the first of many weddings (at least the first that went through to the end after Rachel's flight from Barry). In a busy episode, the couple tied the knot in a ceremony at Monica's apartment after Carol's parents refused to attend - compelling her to reconsider before Ross' unlikely intervention - and some weighty issues (of acceptance mostly) were dealt with in a mature, engaging way, while Phoebe was possessed by a ghost in the background to maintain some laughs.
